This invention relates to improvements in investing casting of directionally solidified eutectic and superalloy components, and more particularly to alumina shell molds and cores for use in the casting of such superalloy components and a method for making such alumina shell molds.
Turbine blades and nozzles that are used in the newest gas turbines must be cast to accurate dimensions with tight tolerances. Of particular interest is the production of single crystal and directionally solidified castings with precise dimensional control of the thickness and dimensions of the metal wall that separates the cooling gas within the turbine blade from the hot gas stream. The dimensional control of the metal wall of the casting is predominately controlled by the properties of the ceramic, known as a core. In this respect, it is important to be able to manufacture a core, to dimensional precision, corresponding to the dimensions of the desired metal casting.
In addition to requiring dimensional precision in the casting of the ceramic core, the production of the above-described directionally solidified metal eutectic alloys and superalloys requires that the core not only be dimensionally stable but also sufficiently strong to contain and shape the casting. In addition, such core must also be sufficiently durable and deformable to prevent mechanical rupture (hot cracking) of the casting during cooling and solidification. Further, the core materials must be able to withstand superalloy casting temperatures of 1500.degree. C. to 1650.degree. C.
The prior art includes the use of silica or silica-zircon (cristobalite) as core and mold materials. Dimensional control of the silica core is difficult for at least two reasons. First, crystalline-based silica materials for the core material are susceptible to Martensitic-type phase changes during the casting process. Accordingly, as a practical matter, cores made of such materials cannot be completely sintered before use in casting. Otherwise, the core may crack once it is cooled down while still in the associated mold. Secondly, thermal expansion differences between the silica core and the associated mold are typically very large. Accordingly, it is difficult, if not impossible, to tightly fix the silica core within an associated mold without rendering the silica core susceptible to cracking.
Aluminum oxide, or "alumina", by itself, without a chemical or physical binder material, has also been identified as a potential core and mold material based on both chemical compatibility and leachability considerations. Unfortunately, ceramic materials comprised of alumina composites are known to be susceptible to excessive shrinkage during firing and have higher than desired fired densities. Such shrinkage is unacceptable for applications where dimensional precision is required during manufacture, such as in the production of directionally solidified metal eutectic alloys and superalloys.
Further, shrinkage with a concomitant decrease in porosity results in a ceramic article with unsuitable mechanical properties for the casting of superalloys. In this regard, because there generally is a considerable thermal expansion mismatch between the ceramic and the alloy, hoop and longitudinal tensile stresses are experienced by the alloy upon cooling from the superalloy casting temperature. Accordingly, if the ceramic article is very dense (i.e., non-porous) with little plasticity and having a high resistance to deformation at elevated temperatures, this can lead to mechanical rupture or hot tearing of the alloy in the ceramic article.
U.S. Pat. No. 4,164,424 discloses low shrinkage ceramic cores made of alumina which posses sufficient porosity and are, therefore, suitable for use in investment casting of directionally solidified eutectic and superalloy materials. A reactant fugitive filler material, which can include aluminum, is mixed with alumina compact to form a green product. The green product is then subsequently fired at an elevated temperature under a reducing (for example, hydrogen gas) or inert atmosphere, whereby reactant material in the fugitive filler material reduces a portion of the alumina which, in part, is removed from the compact in the gaseous state. Some of these gases are deposited on other alumina grains by vapor phase transport action causing a coarsening and rounding thereof, and creates a network of narrow connecting bridges between the alumina grains. With the formation of coarser particles, shrinkage of the ceramic core is mitigated, enabling manufacturing of such core with dimensional precision. Further, sufficient porosity is maintained such that the core does not develop excessive strength.
Unfortunately, the process associated with making ceramic articles disclosed in U.S. Pat. No. 4,164,424 is relatively expensive, requiring furnaces with precision control of water vapor pressure. Further, where the controlled atmosphere is hydrogen gas, additional safety precautions must be taken due to the explosive nature of such atmosphere. Furthermore, the forming process associated with the intermediate green product in U.S. Pat. No. 4,164,424, namely, high pressure injection molding, typically requires costly equipment, which requires frequent maintenance, for example, die wear, rendering fabrication commercially unattractive.
The process disclosed in U.S. Pat. No. 4,164,424 is further hindered by the use of wax binders. Wax binders are typically employed for enhancing the strength of the intermediate green product. Removal of such binder during the firing process is very time consuming as it relies on capillary action induced by surface forces surrounding granular particles. In any event, removal of the associated green product must necessarily be slow otherwise such green product may be vulnerable to cracking, slumping or blisters.